You can see the 2x4s that replaced the 1x4s - I cut out the drywall so I could go right to a joist, glued, screwed and bolted the 2x4s on. I carved out a notch for the newel post to fit in (and it was bolted and glued in place)

I was original planning on using a half newel post here, tapconned to the wall. But the wall turns out to be soft, thick plaster and not concrete. Who knew? In any case, this is strong strong strong.